In recent years, saturated hydrofluorocarbons (HFCs) and unsaturated hydrofluorocarbons (HFOs) which have zero ozone depletion potential and lower global warming potential (GWP) have been used as refrigerants for refrigerating machines. Among them, difluoromethane, 1,3,3,3-tetrafluoropropene, 2,3,3,3-tetrafluoropropene, and the like have low GWPs and thus are refrigerants preferred from the viewpoint of environmental problems. International Publication No. WO2006/094303 discloses use of a mixture of the aforementioned refrigerants. In addition, as refrigerating machine oils for the above refrigerants, International Publication Nos. WO2012/026303 and WO2013/062058 disclose ester-based refrigerating machine oils.
On the other hand, Japanese Patent Application Publication No. 2014-224271 teaches the following points of difluoromethane. Specifically, difluoromethane is less stable than other fluorine-based refrigerants, so that it easily degrades when exposed to a high-temperature environment or mixed with air and water, and generates a large amount of acids such as hydrofluoric acid when degraded. In addition, there are risks that the acids generated by the degradation of the refrigerant may deteriorate the refrigerating machine oil used in the refrigerating machine, and corrode parts such as an expansion valve. Further, in the compressor of the refrigerating machine, difluoromethane is used under higher pressure than the other fluorine-based refrigerants. Accordingly, a large load tends to be applied to sliding portions of the compressor that compresses difluoromethane and the refrigerating machine oil forms a thin film between the surfaces of the sliding parts, with the result that wear and seizure easily occur. In addition, International Publication No. WO2006/094303 mentioned above discloses, as a mixed refrigerant containing difluoromethane, a refrigerant containing a mixture of difluoromethane and an unsaturated hydrofluorocarbon as a main component, but states that the unsaturated hydrofluorocarbon is inferior in thermal and chemical stabilities because it has an unstable double bond in molecules. For this reason, a refrigerating machine oil used together with a mixed refrigerant containing difluoromethane is required to have excellent stability and excellent lubricity.
Meanwhile, as a material for improving the lubricity, U.S. Pat. No. 5,017,300 discloses refrigerating machine oil compositions containing a polyacrylic ester for a HFC refrigerant selected from 1,1,1,2-tetrafluoroethane (R134a), 1,1,2,2-tetrafluoroethane (R134), and pentafluoroethane (R125). Japanese Patent Application Publication No. H04-275397 discloses refrigerating machine oil compositions containing a polyacrylic ester for an R134a refrigerant. However, the aforementioned literatures neither describe a composition containing a polyacrylic ester and an ester of a polyhydric alcohol with an aliphatic monocarboxylic acid, nor describe use of the compositions as one for a mixed refrigerant comprising difluoromethane. Japanese Patent Application Publication No. 2007-204568 discloses refrigerating machine oil compositions for a carbon dioxide refrigerant containing a base oil and an oil-soluble polymer. However, the foregoing literature does not describe use of the aforementioned refrigerating machine oil compositions as one for a mixed refrigerant containing difluoromethane. In addition, Japanese Patent No. 3051673 discloses refrigerating machine oil compositions a Michael addition product of an acrylic ester and a malonic acid ester, and an ester oil.